Currently there is a strong trend toward the development of very low cost RFID tags, relatively small scale integrated circuits whose input and output, via radio frequency radiation, uses transponders rather than receivers and transmitters, which are cheap enough to achieve ubiquitous deployment on essentially any object whose location and movement are of interest. Such tags would simplify and expedite many tasks, including keeping track of and streamlining inventories, preventing theft and counterfeiting, guaranteeing product authenticity, and monitoring environmental conditions experienced by the object. Preferably they should be available in the form of labels which can be easily attached to objects. Some objects, such as secure papers (for example currency, identification documents (including tickets), and certificates or other legal documents) are thin and flexible, and any electronics attached to them should be unobtrusive and preferably invisible.
In many cases, it is desirable for the tags to include some memory beyond the minimal amount required for simple unique identification of the object. For example, a tag may contain information regarding a person's medical history or special needs, or it might contain extensive information regarding proper handling procedures of a product. Memory requirements will vary widely, but often are in the range of 1 kilobyte (kB) to 10 kB, although they may be more or less than these values; some tags are offered with 256 bytes of memory, and some with 256 kB.
Current products do not adequately address these needs. Conventional silicon chips are thick (0.5 mm before thinning), brittle, and too expensive to meet the <5¢ per tag cost target that most customers quote. The cost of finished CMOS ICs has been approximately $4/cm2, or 4¢/mm2, for many years, and while costs per transistor continue to decrease with device scaling, areas costs do not. In addition to the basic chip cost, one must include thinning, dicing, placement, and wire-bonding to an antenna. Even after this, the product has problems, including the fragility of the chip-antenna connection and remaining thickness of silicon. It will be very difficult to incorporate such devices unobtrusively into paper documents.
Another major difficulty with existing solutions is that conventional silicon nonvolatile memory (which comes in two closely related varieties: EEPROM, or electrically erasable and programmable read-only memory, and FLASH memory) is substantially more expensive than CMOS logic circuitry, and requires a different IC process, so that it is often not fabricated in the same chip or at the same time or using the same fabrication processes as logic. Thus, to meet the need for memory described previously, it might be necessary to incorporate another chip, with some suitable connection to the processor. Such connections substantially increase the cost of the product, as well as lowering reliability.
Such nonvolatile memory chips are not only more expensive per unit area than logic chips or volative memory such as DRAM and SRAM, but they cannot be easily scaled to small sizes such as are desired for RFID tags (as well as other products, such as smart cards). Ignoring for the moment the support circuitry needed for addressing the memory, FLASH requires one transistor per bit, and so 1 kB (8 kbits) requires 8000 transistors, which in a modem (e.g. 0.13 micron design rule) integrated circuit process will occupy a square less than 50 microns on each side, which is a very small chip that cannot be produced without greatly increasing the price per unit area relative to the basic production cost of ordinary memory chips. Thus, one is again confronted with the difficulty of getting the amount of circuitry needed at an acceptable price.
Scalable memory materials better suitable for this application might be organic memory materials such as a Zn porphyrin photoconductive thin film disclosed in Liu et al., U.S. Pat. No. 5,327,373 (“Optoelectronic memories with photoconductive thin films”, Jul. 5, 1994) and its division U.S. Pat. No. 5,424,974, hereby incorporated by reference. However, Liu et al. discloses the construction of rigid memory cells which are exemplified by devices constructed between glass plates into which the active material must be introduced by capillary action.
A desirable solution to these problems would be an integrated circuit (IC) which can be fabricated directly on thin substrates such as paper or paper-thin plastic, along with appropriate memory, in a single thin-film integrated process so that the antenna connection is made by the simplest possible techniques, with the greatest possible longevity. It is desirable that the memory be available in the same process, in amounts of about 1 kB to 100 kB (more or less), at a cost that is not a major increase relative to the cost of the circuit itself.